1. Field
The presently disclosed subject matter relates to an optical deflector used as a scanner in a projector, a bar code reader, a laser printer, a laser read amplifier, a head-up display apparatus or the like.
2. Description of the Related Art
Recently, in a projector or the like, a spotlight is deflected by an optical deflector and then, is projected on a screen. Such an optical deflector is a micro electro mechanical system (MEMS) device manufactured by semiconductor manufacturing processes and micro machine technology.
FIG. 1 is a perspective view illustrating a first prior art optical deflector viewed from its rear surface (see: FIG. 16 of JP2001-249300A). In FIG. 1, an optical deflector, which is of a one-dimensional type, is constructed by a square mirror 1, a support frame 2 surrounding the mirror 1, a pair of torsion bars 3-1 and 3-2 connected between the support frame 2 and the mirror 1, and two pairs of piezoelectric actuators 4-1, 4-2 and 5-1, 5-2 provided between the support frame 2 and the torsion bars 3-1 and 3-2 for vibrating (rocking) the mirror 1 through the torsion bars 3-1 and 3-2 with respect to an X-axis of the mirror 1. In this case, both of the mirror 1 and the torsion bars 3-1 and 3-2 are very thin, while the support frame 2 is much thicker than the mirror 1 and the torsion bars 3-1 and 3-2.
When a simulation using the Oofelie-Multiphysics V4.4 (trademark) simulation software provided by Open Engineering was performed upon the optical deflector of FIG. 1, a stress distribution as illustrated in FIGS. 2A and 2B was obtained in the optical deflector of FIG. 1. In FIGS. 2A and 2B, the stronger stress (in this case, Mises stress), whether it is a compression stress or a tension stress, is shown as darker in the illustration.
In FIGS. 2A and 2B, the stress distribution can be represented by a rear-side stress distribution DR viewed from the rear surface of the optical deflector and a front-side stress distribution DF viewed from the front surface of the optical deflector. That is, as shown in FIG. 2B, the rear-side stress distribution DR was symmetrical to the front-side stress distribution DF with respect to a center face therebetween. If this condition is defined as Condition I, the stress distribution of FIGS. 2A and 2B satisfied Condition I.
Determination of whether or not Condition I is satisfied is carried out as follows: A predetermined area is divided into a plurality of grid-shaped cells. Then, it is determined whether or not the difference between a stress within one cell on a rear-side surface and a stress within one cell on a front-side surface corresponding to the one cell on the rear-side surface is smaller than a threshold value. Then, the number of cells whose difference is smaller than the threshold value is calculated. Then, it is determined whether or not the ratio of the calculated cell number is smaller than a predetermined ratio. Finally, when the ratio of the calculated cell number is smaller than the predetermined ratio, Condition I is determined to be satisfied.
Also, the slope of the stress in the rear-side stress distribution DR at a coupling region C between the mirror 1 and the torsion bar 3-1 was smaller than a predetermined value. This condition is defined as Condition II which can be used instead of the above-mentioned Condition I.
Further, the maximum stress in the rear-side stress distribution DR at the coupling region C was smaller than a predetermined value. This condition is defined as Condition III.
Since the optical deflector of FIGS. 2A and 2B satisfied Condition I (or II) and Condition III, it is determined that the mirror 1 and the torsion bar 3-1 at the coupling region C would not break down even when the deflection angle of the mirror 1 is large.
In the above-described first prior art optical deflector, however, as shown in FIGS. 2A and 2B, the relatively large stress was spread isotropically and broadly into the mirror 1 along the X-axis and the Y-axis due to the thin structure thereof. As a result, the mirror 1 per se would warp, so that the optical characteristics of reflected light from the mirror 1 would deteriorate, and at worst, the mirror 1 entirely would break down.
In a second prior art optical deflector (see: JP07-092409), the mirror is much thicker than the torsion bars to reinforce the mirror. That is, the spread of the relatively large stress into the mirror can be suppressed by the thick structure thereof to avoid the warping of the mirror.
In the above-described second prior art optical deflector, however, the weight of the mirror is such as to require larger drive voltages for piezoelectric actuators or the like.
In a third prior art optical deflector, a rib is formed at an outer circumference of a rear surface of a thin mirror in the vicinity of a torsion bar (see: FIG. 2 of JP2010-128116A) or at the entire outer circumference of the rear surface of the mirror (see: FIG. 3 of JP2010-128116A). As a result, while the reinforcement of the mirror is retained, the weight of the mirror is suppressed, which would not require larger drive voltages for piezoelectric actuators or the like.
FIG. 3A is a perspective view illustrating a mirror and a torsion bar of the third prior art optical deflector, and FIG. 3B is a cross-sectional view taken along the line B-B in FIG. 3A. In this case, the portions of the optical deflector of FIGS. 3A and 3B other than the mirror 1 are the same as those of FIG. 1.
In FIGS. 3A and 3B, a rib 1a is formed at an outer circumference of the mirror 1, so that the mirror 1 and the torsion bar 3-1 at their coupling region C would break down. This is discussed below.
When a simulation using the Oofelie-Multiphysics V4.4 (trademark) simulation software provided by Open Engineering was performed upon the optical deflector of FIGS. 3A and 3B, a stress distribution as illustrated in FIGS. 3A and 3B was obtained. That is, the spread of a relatively large stress into the mirror 1 is suppressed by the rib 1a. 
In the above-described third prior art optical deflector, however, a rear-side stress distribution DR was asymmetrical to a front-side stress distribution DF to generate a large asymmetrical region AS illustrated in FIG. 3B. Also, a large slope of stress at the coupling region C would be generated as indicated by Sa. Further, the maximum stress in the rear-side stress distribution DR is large as indicated by Ma. Therefore, since the optical deflector of FIGS. 3A and 3B did not satisfy Condition I (or II) and Condition III, the mirror 1 and the torsion bar 3-1 at the coupling region C would break down even when the deflection angle of the mirror 1 is small (see: θx=8° of FIG. 8).
Another prior art optical deflector forms a plurality of recessed portions within a thick mirror (see: JP2001-249300A). As a result, while the reinforcement of the mirror is retained, the weight of the mirror is suppressed, which would not require larger drive voltages for piezoelectric actuators or the like. However, this prior art optical deflector has the same problems as in the above-described third prior art optical deflector.